1. Field of the Invention
The present invention relates to a semiconductor device and manufacturing method thereof. More particularly, the present invention relates to a technique for improving a withstanding voltage of operation of a semiconductor device while the drive capacity of the semiconductor device is being prevented from deteriorating.
2. Description of the Related Art
FIG. 10 is a cross-sectional view for explaining a conventional semiconductor device.
In FIG. 10, reference numeral 51 is a first conductive type semiconductor substrate, for example, reference numeral 51 is a P type semiconductor substrate. On the substrate 51, the gate electrode 53 is formed via the gate oxide film 52. Source drain regions of one side LDD (Lightly Doped Drain) structure are formed adjacent to the gate electrode 53. That is, on the source region side, the source region 55 of high concentration (N+ type) is formed adjacent to the gate electrode 53. On the drain region side, the drain region 54 of low concentration (N− type) is formed adjacent to the gate electrode 53, and the drain region 56 of high concentration (N+ type) is formed in the drain region 54 of low concentration. The conventional semiconductor device has the source and the drain region of one side LDD structure composed in the manner described above.
In the above semiconductor device of one side LDD structure in which a high voltage is impressed only upon the drain region side, in order to prevent an electric field from concentrating upon the drain region side, it is composed in such a manner that the drain region 56 of high concentration is surrounded by the drain region 54 of low concentration and only the source region 55 of high concentration is formed on the source region side in which a high withstanding voltage is unnecessary.
Even in the semiconductor device of the above structure, no problems are caused with respect to the static withstanding voltage. However, in the case of operation, the following problems are caused in the semiconductor device of the above structure.
The problems are described as follows. In the bipolar structure composed of a source region (emitter region), substrate (base region) and drain region (collector region), since the source region 55 of high concentration is exposed from the emitter region, the injection efficiency of carrier is high, so that the bipolar transistor is easily turned on by a low intensity of substrate electric current (I sub).
That is, since electric current gain β is high in the bipolar transistor, the withstanding voltage of drain is lowered at the operation time compared with the semiconductor device of both side LDD structure.
In this case, if a commonly used both side LDD structure is adopted, electric current gain β is lowered and it is sure that the withstanding voltage is enhanced. However, although high withstanding voltage is originally unnecessary on the source side, the common LDD structure is adopted on the source side, too. Therefore, the source side necessarily has the same length (L) of the drift region as that on the drain side. Accordingly, ON-resistance is increased and the drive capacity is lowered.